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Dolby Pro Logic Surround Sound Decoder, Pt.2 In this second and final article, we describe construc­tion and testing which involves assembly of the PC boards and a fair amount of interconnecting wiring. By JOHN CLARKE The Prologic Surround Sound Decoder and Effects Unit is housed in a low-profile metal case measuring 430mm wide, 59mm high and 307mm deep, including knobs, rubber feet and the rear heatsink. Virtually all of the components, with the exception of the power transformer, switches and potentiometers, are mounted on PC boards. There are five boards in all: the main decoder board, labelled “Pro Logic Main”, 70  Silicon Chip code 01409951, 160 x 165mm; the power supply, code 01409952, 105 x 140mm; the power amplifier board, code 01409953, 200 x 50mm; the microprocessor board, labelled “Pro Logic Micro”, code 01409954, 76 x 90mm; and the display board, code 01409955, 26 x 115mm. Begin construction by checking the PC boards for any de­fects. Check particularly for any breaks or shorts between tracks. There should be 3mm holes on all boards for the mounting screws and a 3mm hole is required to accommodate the regulator mounting screw for REG5 on the power supply board. Start assembly of the main decoder PC board (see Fig.4) by inserting all the PC stakes required for external wiring and then the links, using tinned copper wire. To produce a neat job with the links, we recommend that the wire be slightly stretched: grip one end of a length of wire (say 30cm long) in a vise and then pull the other end with a pair of pliers. Pull just hard enough to make the wire “give” slightly and then it will become straight. Cut the wire to lengths suitable for each link and bend the ends of each link using pliers so that they fit neatly into the required positions. Next, install the ICs. Take care with their orientation, noting that IC1 and IC2 face in different directions, while R OUT 100k 100  EFFECTS AMP IN GND EFFECTS AMP OUT GND L OUT 100k 100 GND 100 SURR OUT SURR OUT C OUT 100k GND GND 100 RELAY +25V 180pF 4.7k 4.7k VR3 D11 RLY2 RLY4 RLY1 0.33 33k RLY3 1 100 IC5 LF347 47k 0.1 7.5k 1 IC4 LF347 -15V 47k 47k 47k 47k 100k Q1 180pF 0.1 VR2 100 4.7k 470 10uF PC0 +15V .068 100pF 47uF 15k 18k 470pF 10uF LL 680pF .0033 .047 0.68 680pF 0.22 0.22 1 2x.022 0.22 0.22 4.7uF VR4 1uF 470pF 0.1 15k 1uF 4.7uF +4V 10M 0.1 .047 IC1 M69032P 0.1 .0056 15k 330k 0.1 0.1 10  15k .068 .0022 100uF 10  E 0.1 1M X1 22k 100k 22k 22k A 8.2k 100k 15k IC2 M65830P 470pF B +12V 7.5k 18k 5.6k 30  1 .0047 22uF 47k 7.5k 0.1 15k 15k 68k 68k 22k 100uF 0.1 10uF 10uF 47k 7.5k 0.1 1uF 1uF .0056 .0056 100uF 22k 1uF 10uF 25VW GND .056 L IN G R IN 0.1 1uF GND 2.7k 22k 1uF 1uF LP OUT 22k .047 1k 0.1 1uF IC3 TDA1074A +20V 100pF 100  10uF 100uF 10uF 0.18 1 R S P 15k 22k 100uF 1uF 0.1 1uF VR1 150k 0.22 15k 68k 180pF 68k 180pF 7.5k 7.5k 15k 15k GND 0.1 15k +5V 15k 15k GND Fig.4: the component overlay diagram for the main decoder board. Take care to avoid solder bridges between the closely spaced pins of IC1 and check that all polarised parts are correctly oriented. IC3, IC4 & IC5 all face in the same direction. When soldering the closely spaced pins of IC1, be sure that solder does not bridge between pins. When installing the resistors, check the colour code for each value against Table 1. It is also a good idea to check each value with a digital multimeter. When inserting the capacitors, use Table 2 to check the values. For example, a .047µF capacitor could be labelled 47n or 473. Once the capacitors are in, mount the four reed relays, diode D11, transistor Q1 and the 2MHz crystal, X1. Take care with the orientation of D11 and Q1. Amplifier board assembly Refer now to Fig.5 for the component overlay of the power amplifier board. Again, start with the PC stakes and the link (one only), then insert the resistors and capacitors. The fuse holder clips are inserted with their locating tabs oriented toward the ends of the fuse. If the tabs are located incorrectly, you will not be able to fit the fuses. The power ICs (IC7, IC8 and IC9) come with preformed leads. When inserted and soldered, the mounting hole in each metal tab should be located 16mm above the PC board, to line up with the holes in the rear of the chassis. The power supply board is equally straightforward (see Fig.6) and you can start with the PC stakes and links. This done, install the diodes, taking care with the orientation of each. Note that D1-D4 are larger than D5-D10. The small bridge, BR1, must be located with its notched end adjacent to the 470µF capacitor. The three 0.25W resistors can be mounted next, followed by the four 3-terminal regulators, REG1-4. Make sure that you don’t mix December 1995  71 47uF 25VW Surround Sound Decoder – ctd D5 +25V TO RELAYS D6 18VAC GND 0V D1-D4 GND GND 1k 22uF F3 CENTRE IN 18VAC 22k 2.2uF 100uF 10k 10000uF 25VW +25V GND 10000uF 25VW -25V IC7 18k 0.1 0.1 100uF D10 D9 D8 D7 680W 5W 0.22 1 4700uF 25VW TO CENTRE SPEAKER F2 +20V 1000uF +12V SURR L IN GND 2.2uF 100  5W REG5 7805 18k 0.1 0.1 +5V +5V SURR R IN 1k X2 1M 2.2uF B A E R S D PC0 2x39pF 10k IC6 MC68HC705C8P 18k 0.1 1k 0.1 IC9 0.1 47k 47k 47k 22k 100uF DIP1 ON 47k 0.1 GND 10uF Fig.6: the component overlay for the power supply board Note that the diodes for D1-D4 are larger than those for D5-D10. TO SURR L SPEAKER 10uF 100uF 0.22 F6 1 10k GND 1 Fig.5: the component overlay diagram for the power amplifier board. No setting-up adjustments are required for the power amplifiers. +5V S4b +5V 330 330 330 330 330 330 330 330 330 330 GND 330 330 330 TO SURR R SPEAKER 72  Silicon Chip +15V 3x10uF 0.22 10uF F7 470uF 1.8k 1 22uF 470uF BR1 100uF F4 REG3 -15V 330  GND REG4 2x 10uF IC8 -25V +25V REG1 120  22k 100uF F5 REG2 1k 22uF Fig.7: the component overlay for the microprocessor board. We used a 6-way pin header for the B, A, E, R, S and D output lines. TABLE 1: RESISTOR COLOUR CODES 4-Band Code (1%) brown black blue brown brown black green brown orange orange yellow brown brown green yellow brown brown black yellow brown blue grey orange brown yellow violet orange brown orange orange orange brown red red orange brown brown grey orange brown brown green orange brown brown black orange brown grey red red brown violet green red brown green blue red brown yellow violet red brown red violet red brown brown grey red brown brown black red brown yellow violet brown brown orange orange brown brown brown red brown brown brown black brown brown orange black black brown brown black black brown brown black gold gold D13 D12 S5 LED1 10k S6 D14 10k DISP2 HDSP5301 10k DISP1 HDSP5301 Value 10MΩ 1MΩ 330kΩ 150kΩ 100kΩ 68kΩ 47kΩ 33kΩ 22kΩ 18kΩ 15kΩ 10kΩ 8.2kΩ 7.5kΩ 5.6kΩ 4.7kΩ 2.7kΩ 1.8kΩ 1kΩ 470Ω 330Ω 120Ω 100Ω 30Ω 10Ω 1Ω 10k ❏ No. ❏   3 ❏   2 ❏   1 ❏   1 ❏   6 ❏   4 ❏ 11 ❏   1 ❏ 12 ❏   5 ❏ 14 ❏ 11 ❏   1 ❏   6 ❏   1 ❏   3 ❏   1 ❏   1 ❏   5 ❏   1 ❏ 14 ❏   1 ❏   7 ❏   1 ❏   2 ❏   3 A K S7 Fig.8: the display board. The two 7-segment displays are oriented with the decimal points to the lower righthand side, while the switches (S5-S7) all have their flat side towards the top of the board. LED1 should initially have only one lead soldered to the board to allow for easy adjustment later on. them up. REG1 is a 7815 type, REG2 is a 7915, REG3 is a 7812 and REG4 is an LM317. REG5 is mounted on a small heatsink using a screw and nut to secure it to the PC board. The capacitors are next. The two 10,000µF and 4700µF electrolyt­ics are mounted on their side and can be secured to the PC board using a small amount of silicone sealant. The remaining capacitors are mount­ ed vertically, with the polarity shown. The 100Ω and 680Ω 5W resistors are mounted 1mm proud of the PC board to allow cooling. Microprocessor board The microprocessor board has only a few parts, as shown in Fig.7. We used a 6-way pin header for the B, A, E, R, S and D output lines. Make sure you orient IC6 correctly. Its notched end is adjacent to the 10µF capacitor. The DIP switch, DIP1, is oriented with the “on” label adjacent to the edge of the PC board. The display PC board is next – see Fig.8. Solder in the resistors and diodes, taking care with the orientation of D12-D14. The two 7-segment displays are oriented with the decimal point to the lower righthand side, while the switches, S5-S7, have the flat side toward the top of the PC board. Finally, install LED1 and solder only one lead to 5-Band Code (1%) brown black black green brown brown black black yellow brown orange orange black orange brown brown green black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown orange orange black red brown red red black red brown brown grey black red brown brown green black red brown brown black black red brown grey red black brown brown violet green black brown brown green blue black brown brown yellow violet black brown brown red violet black brown brown brown grey black brown brown brown black black brown brown yellow violet black black brown orange orange black black brown brown red black black brown brown black black black brown orange black black gold brown brown black black gold brown brown black black silver brown TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value IEC Code EIA Code 0.68µF   680n   684 0.33µF   330n   334 0.22µF   220n   224 0.18µF   180n   184 0.1µF   100n   104 .068µF   68n  683 .056µF   56n  563 .047µF   47n  473 .022µF   22n  223 .0056µF   5n6  562 .0047F   4n7  472 .0033µF   3n3  332 .0022µF   2n2  222 680pF   680p   681 470pF   470p   471 180pF   180p   181 100pF   100p   101 39pF   39p   39 the PC board. This will allow easy adjustment later on. The display and microprocessor boards are soldered together at right angles after aligning the track buses together. At this stage only tack solder December 1995  73 CENTRE SURR L SURR R IEC PLUG F1 BROWN (ACTIVE) GND NEUTRAL (BLUE) EARTH GREEN/ YELLOW EARTH LUG GND 2 1 MOV POWER AMPLIFIERS ORANGE ORANGE WHITE PINK YELLOW T1 RED 26 25 B E S A R D POWER SUPPLY 26 MICROPROCESSOR CONTROL DISPLAY .01 3kV S7 6 10M S1 S6 10M S5 74  Silicon Chip 8 9 S4 S3 7 VR1 SUBWOOFER L OUT OUT L IN 0.47 R IN Fig.9: this diagram shows the general disposition of all the PC boards. Be sure to run shielded audio cable in the locations indicated and use mainsrated cable for all mains wiring to the IEC plug, transformer, power switch and fuseholder. R OUT at two locations so that the boards can be adjusted when installed in the case. GND GND 3 Chassis assembly GND +25V -25V 4 5 1 MAIN 2 25 10 23 24 9 12 14 7 6 8 11 13 16 15 17 20 22 19 21 18 B A E S R D 24 23 VR2 VR3 13 15 16 21 22 20 14 10 12 11 3 19 17 18 5 4 S2 VR4 Work can now begin on the case. The general disposition of all the boards and the interconnecting wiring is shown in Fig.9. First, secure the sides to the baseplate using the self- tapping screws supplied. This done, cut the pot shafts and rotary switch shaft to a length suitable for the knobs supplied. Install these and switches S1, S3 and S4 on the front panel. Also insert the red Perspex window for the 2-digit display. Next, affix the Dynamark labels in position on the rear panel and fit the RCA sockets, fuseholder, IEC mains socket and loudspeak­ er terminals. This done, attach the front and rear panels to the chas­sis with the screws supplied. You can now mount the amplifier PC board against the back of the case on seven 6mm standoffs, using 3mm screws and nuts. The three power amplifier ICs are secured to the rear panel with TO-220 insulating washers and insulating bushes. The screws also hold the heatsink in place. Apply a smear of heatsink compound between the mating surfaces of the heatsink and rear of the case before assembly. The Dolby licensing label can now be affixed to the top of the heatsink. Mount the decoder and power supply PC boards on the base of the case as shown on the wiring diagram of Fig.9 using 9mm tapped spacers and short 3mm screws. The microprocessor PC board is mounted on 12mm spacers. Initially, secure the spacers to the PC board so that it can be positioned in the base of the case. Now check that the pushbutton switches are centred in the front panel holes. If necessary, adjust the height by re­soldering the front panel display board. The remaining connections between the two boards can now be soldered. December 1995  75 This photo shows the general layout inside the chassis. Note the use of plastic cable ties to bind the shielded cable runs between the PC boards and the front panel controls. The large heatsink on the rear panel dissipates the heat generated by the three LM1875 power amplifier ICs (IC7-IC9). Next, the micro/display board can be mounted in place. Secure the standoffs to the baseplate and adjust the LED so that it just protrudes through the front panel. Solder both leads to the display PC board. Transformer wiring Bolt the toroidal transformer to the base of the case using the two rubber washers and the large washer. Secure the mains terminal block to the case 76  Silicon Chip as shown in Fig.9. The earth lug is secured to the chassis with screw, nut and star washer. Scrape away the paint or anodising around this screw hole to ensure a good earth contact. Use mains-rated wire for all 240VAC connections. Solder a green/yellow striped earth wire to the earth terminal of the IEC socket and solder it to the earth lug. Using a blue mains rated wire, solder one end to the Neutral side of the plug. This con­nection must be insulated with heatshrink tubing, so slip a length over the wire before securing into the terminal block. Similarly, the brown mains wire secures to the Active terminal of the socket with heatshrink tubing over its terminal. Solder the Active lead to the fuse after slipping a length of heatshrink tubing over the wire, Solder another brown wire to the second terminal of the fuse holder and insulate the fuse terminals with the tubing. Again, switch S1 is insulated with heatshrink tubing after soldering the wires to the terminals. These wires connect to the fuse and terminal block TABLE 3: DIP SWITCH SETTINGS as shown. Do not forget the .01 3kV capacitor across the switch and the varistor (MOV) across the terminal block. The fuseholder, IEC plug and switch insulating tubing can now be shrunk down with a hot air gun. Connect the orange primary transformer wires to the termi­ nal block and solder the secondary wires to the power supply board. You should now carefully check all your work before moving to the test procedure. Testing Insert the fuse into the rear panel holder, fit an IEC mains lead and apply power. Use your multimeter to check that the voltages on the power supply board are correct. These are shown on the board overlay diagram of Fig.6. Note that the +25V rail can be as high as +28V. The regulator output voltages should be within ±5% of their nominal values. If it all checks out, remove the power so that you can continue the wiring for the DC rail connections. We used green hook-up wire for the GND wiring, red for +5V, blue for -15V and yellow for +15V. There is nothing sacred about this but you should use consistent colours for all the wiring. Delay 1 2 3 4 15ms on on on on 16ms on on on off 17ms on on off on 18ms on on off off 19ms on off on on 20ms on off on off 21ms on off off on 22ms on off off off 23ms off on on on 24ms off on on off 25ms off on off on 26ms off on off off 27ms off off on on 28ms off off on off 29ms off off off on 30ms off off off off Other wiring using hook-up wire should be completed now. Note that the wiring between the B, A, E and R, S, D terminals on the microprocessor PC board and the decoder board is done using the two separate 3-way rainbow cables. Terminate the microproces­ sor wire ends into the header socket pins. The wires then pass under the power supply PC board to connect into the R-D termi­nals. The B-E terminal wires also pass under the decoder board. The RCA sockets require a short length of tinned copper wire soldered between each earth connection. A 0.47µF capacitor solders between this wire and a solder lug which is secured to the chassis. Use a multimeter on the Ohms range to check that this lug is properly earthed. The remaining wiring is run using shielded cable. Try to keep these wires as short as possible and use cable ties to bundle them into neat looms. There are two holes in the decoder board to secure a cable tie near switch S2. Do not forget to solder the two 10MΩ resistors across the terminals of switch S3. When the wiring is complete, check your work thoroughly, then apply power and recheck the voltages on the power supply board. If these are now incorrect, switch off immediately and check for wiring errors. If the voltages are correct, observe December 1995  77 TABLE 4: PERFORMANCE OF PROTOTYPE Dolby Requirement Prototype -3dB <at> 50Hz & 15kHz; R & L channels -3dB <at> 50Hz & 6-8kHz; S channel -3dB <at> 50Hz & 15kHz; wideband C channel -3dB <at> 90-140Hz & 15kHz; normal C channel -3dB <at> 15Hz & 40kHz -3dB <at> 24Hz & 7kHz -3dB <at> 20Hz & 40kHz with C trim centred -3dB <at> 120Hz & 40kHz with C trim centred -65dB CCIR/ARM L, C & R channels -71dB unweighted Distortion <1% <at> 300mV input & 1kHz 0.05% R, L & C outputs; 0.15% S output Headroom +15dB above reference; all channels 17dB S ouput; 15.5dB R, C & L outputs <350mV RMS 300mV RMS 25dB minimum between channels >31dB between channels Volume Tracking <3dB over top 40dB range for all outputs <0.2dB to -70dB level; <1dB to -80dB S Channel Delay 20ms fixed or 15-30ms 15-30ms adjustable Noise Sequencer 10-15db below reference -11.3dB <at> 2s/channel 2V RMS 5.6V RMS ±10dB for C & S channel outputs ±10dB -3dB <at> 90-140Hz -3dB <at> 130Hz Frequency Response S/N Ratio (wrt to 100mV or 1W into 8W) Input Sensitivity Crosstalk Output Clipping Gain Trim Subwoofer Output Power Output 20W RMS per channel into 8W load Note: reference level is 300mV/1kHz at pin 30 of IC1 (C out) the LED display. At switch-on, the display will show two dashes (- -), then after about five seconds the display will show a delay time between 15 and 30 seconds. The actual time will depend on the settings of DIP switch DIP1. Table 3 shows how to set DIP1. Delay values can be altered using the UP and DOWN switches. Pressing the Noise switch will change the display to show L, C, r and S in sequence. The LED will also light. Note that the Mode switch must be in the surround position for all four display indications. The 3-stereo and stereo settings will truncate the display settings to L, C and r and L and r accordingly. Check that the relays switch on at the instant the LED display changes from the dashes to the delay time at switch on. They produce a slight clicking sound when closing. Check that +12V is present at pin The microprocessor and display boards are butted together at rightangles to form a single assembly before mounting in the chassis. 78  Silicon Chip 37 of IC1 and +20V is at pin 11 of IC3. Pin 4 of IC4 and IC5 should have +15V while pin 11 of these ICs should have -15V. IC7, IC8 and IC9 should have +25V on pin 5 and -25V on pin 3. There should be +5V at pins 1 and 24 of IC2 and pins 40, 37, 34 and 3 of IC4. Check also for +4V at pins 43 and 44 of IC1. A +10V reference should be at pin 8 of IC3. Connect a stereo amplifier to the left and right channel outputs and This photo shows how the leads to the fuseholder and IEC socket are fitted with heatshrink sleeving to prevent accidental contact with the mains. loudspeakers to the centre, surround left and sur­round right amplifier outputs. Switch on the noise sequencer with the Mode switch in Surround mode. Check that there is a noise signal in each channel. Adjust the surround and centre trim controls so that there is equal volume in all channels. Check the volume control operation from minimum to maximum rotation. At minimum volume, nothing should be heard from the loudspeakers while at maximum volume it should be loud. If all is well, you can connect up to your stereo TV or stereo VCR. The left and right channel outputs from your VCR or TV connect to the left and right channel inputs of the Surround Sound Decoder. It is important not to cross-connect the left and right channels otherwise the decoder cannot operate correctly. For the centre loudspeaker, there are several options available. Firstly, no loudspeaker is required if the phantom mode is selected. The centre channel signal will be diverted equally into the left and right channels. The second approach is to use a centre channel speaker which does not have bass response below 100Hz. When the normal selection for the centre channel is selected, signals below 100Hz are rolled off in the centre channel and added to the left and right The LM1875T power amplifiers (IC7,8,9) are each secured to the rear panel with a TO-220 mounting kit, to isolate them from chassis. The three screws also secure the heatsink to the rear of the chassis. *Trademarks & Program Requirements Note 1: “Dolby”, “Pro Logic” and the Double-D symbols are trademarks of Dolby Laboratories Licensing Corporation, San Francisco CA94103-4813 USA.) Note 2: this Dolby Pro Logic surround sound decoder requires a program source such as a stereo TV set or hifi stereo VCR. The program must be Dolby Surround encoded as depicted in the movie credits by the Dolby double-D surround symbol. For unencoded stereo signals, the Dolby 3-stereo selection will provide the centre front channel. Effects selection will provide surround sound from any stereo signal source. The decoder will not operate from a mono signal. December 1995  79 The rear of the chassis has a large single-sided heatsink for the power amplifiers, RCA sockets for the inputs and front chan­nel outputs, three pairs of terminals for the centre and rear speakers, and an IEC power socket. AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 12 OCT 95 11:41:12 1 0.1 0.010 0.001 T .0005 20 T T 100 1k AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 10k LEVEL(W) 20k 12 OCT 95 11:37:41 1 channels. As a consequence, the centre bass information is not lost. Warning! If a centre loudspeaker is used, it must have magnetic shielding if it is to be placed on top of or underneath your TV set. Severe colour distortions and loss of purity could result from placement of a normal speaker near a television screen or monitor. The third alternative is to use a full range loudspeaker in the centre channel. In this case, the wideband selection is chosen for the centre channel. The subwoofer output can be connected to an amplifier and loudspeaker which can provide a low frequency bass response. Note that this option is available only for the phantom and normal settings for the centre loudspeaker. When listening to Dolby encoded video tapes, the Dolby Prologic setting should be used. Adjust the delay time for best results. For unencoded music, the Effects setting will provide a rear channel ambience. The effects control sets the amount of rear channel level, while the delay can be adjusted to provide the required amount of echo. Errata Two errors have appeared in the parts list published last month. First, the capacitor across the mains switch should be .01µF/3kV, not 0.1µF. Second, there are eight 22kΩ resisSC tors, not seven. 0.1 0.010 Kit Availability 0.001 .0005 0.1 1 10 50 Figs.10 & 11: these two diagrams show the performance of the three power amplifiers. At top is the harmonic distortion versus frequency at a power level of 10 watts while immediately above is the harmonic distor­tion versus power at 1kHz. 80  Silicon Chip Kits will be available from all Jaycar Electronics stores. Our thanks to Jaycar Electronics for their assistance in the development of this project and for their liaison with Dolby Laboratories who have approved the design. Jaycar Electronics is the licensee for the design which was developed in our labora­tory.